The present invention relates to air intake controllers and control methods for internal combustion engines that perform stratified charge combustion. More particularly, the present invention pertains to air intake controllers and control methods for engines provided with swirl control valves that are opened and closed to produce a swirling flow of air-fuel mixture.
In a typical engine, fuel is injected into an intake port by a fuel injection valve to charge the associated combustion chamber with a homogeneous mixture of fuel and air. In the engine, an air intake passage is opened and closed by a throttle valve, which is operated in cooperation with an acceleration pedal. The opening of the throttle valve adjusts the intake air amount (and ultimately the amount of uniformly mixed air and fuel) that is supplied to the combustion chambers of the engine. This controls engine power.
However, when performing homogeneous charge combustion, the throttling action of the throttle valve drastically decreases the pressure in the intake passage. This increases pumping loss and decreases efficiency. Stratified charge combustion solves such problems. In stratified charge combustion, the throttle valve is opened wide, and fuel is supplied directly into each combustion chamber. This delivers a rich, highly combustible air-fuel mixture to the vicinity of the spark plug and enhances ignitability.
Japanese Unexamined Patent Publication No. 6-93943 describes an engine that produces a swirling flow of air-fuel mixture, which includes the injected fuel, to perform stratified charge combustion. The force of the swirl is controlled by adjusting the opening angle of a swirl control valve (SCV). Furthermore, in this engine, the ignition timing and fuel injection timing with respect to the crank angle is retarded to prevent misfires in the cylinders, especially when the engine is in a cold state.
However, the SCV angle and the swirl force differ between engines. The clearance between the air intake passage and the SCV, the fuel spray, and the required swirl force also differ between engines. When performing stratified charge combustion, during which the interval between fuel injection and ignition is short, it is necessary to produce an appropriate swirl to assist the diffusion and vaporization of the fuel.
These factors are not dealt with appropriately in conventional engines. Thus, fluctuation of the output torque differs between engines. In some cases, this may lead to misfires in the cylinders.
There are engines that perform stratified charge combustion and semi-stratified charge combustion. Stratified charge combustion is performed when the engine load is in a low range, while semi-stratified charge combustion is performed when the engine load is between the low and high load ranges, or in a transient range. During the stratified charge combustion, fuel is injected during the latter half of the compression stroke and is concentrated about the spark plug. In this state, the fuel is ignited. During the semi-stratified charge combustion, fuel is injected during the suction stroke and also during the latter half of the compression stroke. Thus, the concentration of fuel about the spark plug is smaller in comparison to stratified charge combustion. In this state, the fuel is ignited.
Japanese Unexamined Patent Publication No. 7-83101 describes an engine that burns a lean air-fuel mixture to perform lean combustion (or lean burn) by swirling the air-fuel mixture, which includes the injected fuel. In this engine, a swirl control valve (SCV) is provided in the air intake passage. The opening angle of the SCV is adjusted to control the force of the swirl. This burns the air-fuel mixture in a satisfactory state during lean combustion and during stoichiometric combustion (combustion of stoichiometric air-fuel mixture).
A target opening angle of the SCV is computed in accordance with the operating state of the engine (e.g., in accordance with the basic fuel injection amount, which is obtained from the depression degree of the acceleration pedal). The SCV is controlled based on the computed target angle. However, in this prior art engine, a delayed response of the valve may result in a difference between the target angle and the actual angle.
The fuel injection timing is generally determined in accordance with the engine speed or engine load. As a result, if the actual SCV angle differs from the target SCV angle, the swirl may become too strong or too weak. This causes the traveling speed of the injected fuel (air-fuel mixture) to become faster or slower than the required speed. In such cases, the vaporization time of the fuel may be too long or too short. Furthermore,, the combustible air-fuel mixture may not be delivered to the vicinity of the spark plug. As a result, combustion becomes unstable. Such state may also lead to misfires in the cylinders.
To deal with the differences between the actual SCV angle and the target SCV angle, the fuel injection timing or the ignition timing may be altered. More specifically, if the swirl is too strong and the fuel travels faster than required, the fuel injection timing may be retarded. On the other hand, if the swirl is too weak and the fuel travels slower than required, the fuel injection timing may be advanced. In this manner, the correction of the fuel injection timing enables the optimal vaporization time to be maintained and guarantees the delivery of combustible air-fuel mixture to the vicinity of the spark plug.
In a system that obtains the basic fuel injection amount based on the depression degree of the acceleration pedal, the value of the basic fuel injection amount changes drastically when the acceleration pedal is depressed in a sudden manner. However, when the depression degree of the acceleration pedal changes suddenly, the amount of air and recirculated exhaust gas drawn into the combustion chamber does not increase in correspondence with the sudden fluctuation of the basic fuel injection amount. Thus, injection of the basic fuel injection amount, which is computed from the depression degree of the acceleration pedal, during rapid acceleration or deceleration of the engine, may cause the air-fuel mixture to become rich or lean.
Therefore, in the prior art, the basic injection fuel amount is graded to vary gradually when the depression degree of the acceleration pedal changes suddenly. That is, a graded fuel injection amount is obtained. An amount of fuel corresponding to the graded fuel injection amount is injected from the fuel injection valve. Therefore, the amount of fuel injected from the fuel injection valve optimally corresponds with the delayed increase in the amount of intake air and recirculated exhaust gas. This enables the air-fuel ratio to be maintained at an optimal value.
However, the target angle of the swirl control valve is computed from the basic fuel injection amount. Therefore, when fuel is injected in correspondence with the graded fuel injection amount during a sudden change in the depression degree of the acceleration pedal, the target SCV angle may be inappropriate with respect to the operating state of the engine. This results in inappropriate fuel injection timing or ignition timing, which are corrected based on the difference between the target SCV angle and the actual SCV angle. Thus, it is difficult to stabilize combustion and prevent misfires during sudden acceleration or deceleration.